This proposal concerns the three dimensional arrangement and detailed structural organization of components within the cell nucleus. The nucleus is a complex organelle that contains, in the form of specific DNA sequences, the information needed for cell construction, maintenance, and replication. Although recent advances in molecular biology have revealed many of the details of gene structure and the decoding and transmission of genetic information, knowledge of the spatial framework within which these mechanisms operate has lagged behind. This is impeding a full understanding of the process of gene activation and control. Experiments are proposed to determine the three dimensional structures and spatial interrelationships within the nucleus. Electron microscopy will be combined with tomography and computer image processing to generate 3D reconstructions of nuclear components, with particular emphasis on the DNA- protein complex known as chromatin. Preparative methods shown to perturb the native structures as little as possible will be employed, and the work will be carried out using sources of nuclei that will be employed, and the work will be carried out using sources of nuclei that are most likely to reveal the basic structural principles of chromatin organization.